This is a program project grant involving a systematic study of calcium metabolism and skeletal metabolism in man. The major areas of study are 1) the hormonal regulation of the plasma calcium and phosphorus concentrations; 2) the metabolism of bone, joint, and connective tissue; and 3) the effects of vitamin D on the intestinal mucosa and intestinal calcium absorption. These problems will be investigated in health and disease. Members of the Endocrine, Arthritis, and Gastrointestinal Units, individually and in collaboration, will study control subjects and subjects with abnormalities of bone or bone mineral metabolism. The objective is a more precise, more detailed, and at the same time more integrated picture of normal and abnormal calcium and skeletal metabolism in man.